But I don't really understand why could I change the background color to transparent by renaming this png?

I mean if you create a MobileIcons.frameworks folder within theme\bundles,I think winterboard will just replaces every png in that foler not using the ones in /Privateframeworks/MobileIcons.frameworks .

The only way I can remove this ugly black background is to turn off the auto-skinning function by deleting the folder /bundles/mobileicon.framework . So if you hate this background and be willing to change every single icon manully, delete it.

========================

I tried several times again.

And I found that if you only delete the folder /bundles/mobileiconframework or only delete the mask.png, it works temporarily. (respring)

And if you change to another theme and back, it will use the mask.png in privatemobileicon/ and still there is a ugly black background.

The bright side is that deb-app still got masked correctly by the mask.png in privatemobileicon/ if you only delete the mask.png or the whole folder with bundels/mobileiconframework .

So the problem is still not solved. How can I make the mask background transparent?

I replace the bundles/com.apple.mobileicons.framework/[email protected] with your png and it turned to this:

(the black smoke thing is the shadow png)

(By the way, clearing all the files in cache foler doesn't work. Nothing changed. Your have to switch to another theme and switch back.I got the pic above by doing this.I'm afraid this is because the real caches are in the memory of the phone, not these in the HDD.)

So, just like what I said, I think maybe it is this way only in iphone4 but it really doesn't matter what color the mask.png is. Black or white,they are the same.

black = visible
white = visible
transparent = transparent

like this:

so,however you change the color of the mask.png , the background is still black.

And I also replaced the com.apple.springboard\WallpaperIconShadow.png with your png. And here:

I worked on this for about 2 hours, which was really boring. I got nowhere.

Whoa, just as I had resolved to give up (right after I hit the period-key on the above sentence), I resprung and it's perfect. What the F*#$@?

Ok, so I tried every combination of things suggested in this thread. This is what I did, I think:

1. AppIconOverlay and AppIconShadow are both Transparent inside Bundles/com.apple.mobileicons.framework. I left out the AppIconMask.
2. Go into var/mobile/Library/Caches/com.apple.iconscache and delete everything in that folder.
3. This is where things get hazy: I rebooted, then went into Winterboard and hit the SummberBoard Mode button ON then OFF and resprung.

The icon still gets darker when I press it, but heck I can live with that -- it's time to move on...

Update: It didn't stick. I'm thinking now that Winterboard needs a major overhaul.

That's it, i tested this 20 times on my iPhone4 with iOS 4.1 and it works if you follow these steps exactly. Also everytime you respirng in Winterboard icon backgrounds will become black again and you need to redo all the steps above. Respringing in SBsettings is ok and icons background will stay transparent.
I dont have anymore iPhones to test this so i would appreciate that someone lets me know does it work on their iPhone thanks.